Re: [PATCH v2 6/7] mm, slab: add static key for should_failslab()

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jun 25, 2024 at 7:24 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 6/20/24 12:49 AM, Vlastimil Babka wrote:
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3874,13 +3874,37 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
> >                       0, sizeof(void *));
> >  }
> >
> > -noinline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
> > +#if defined(CONFIG_FUNCTION_ERROR_INJECTION) || defined(CONFIG_FAILSLAB)
> > +DEFINE_STATIC_KEY_FALSE(should_failslab_active);
> > +
> > +#ifdef CONFIG_FUNCTION_ERROR_INJECTION
> > +noinline
> > +#else
> > +static inline
> > +#endif
> > +int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
>
> Note that it has been found that (regardless of this series) gcc may clone
> this to a should_failslab.constprop.0 in case the function is empty because
> __should_failslab is compiled out (CONFIG_FAILSLAB=n). The "noinline"
> doesn't help - the original function stays but only the clone is actually
> being called, thus overriding the original function achieves nothing, see:
> https://github.com/bpftrace/bpftrace/issues/3258
>
> So we could use __noclone to prevent that, and I was thinking by adding
> something this to error-injection.h:
>
> #ifdef CONFIG_FUNCTION_ERROR_INJECTION
> #define __error_injectable(alternative)         noinline __noclone

To prevent such compiler transformations we typically use
__used noinline

We didn't have a need for __noclone yet. If __used is enough I'd stick to that.